How can chromosomes mutate together without detrimental effect?

Question

There are 46 chromosomes in humans and 48 in apes.

Answer 1

You are correct that most ape species have 24 pairs of chromosomes and humans only 23 pairs. The evidence seems to show that this is the result of a fusion of chromosome 2 through a process called "translocation". (The evidences is the fact that we can compare our chromosome 2 with two chromosomes in the chimp, and see the same sequences.) A translocation is just a copy of a sequence of DNA from one chromosome to another ... occasionally this can copy so much of one chromosome onto another that the second one ends up essentially containing all of the DNA from both chromosomes ... it is basically a fusion of the two chromosomes. The other chromosome is then redundant, and may be lost without detrimental effect during subsequent fertilization. The organism then has one fewer chromosome, but notice that it has the SAME DNA, the SAME genes.

[BTW, technically this is not a "mutation" (so it is incorrect to say they "mutated together") as a mutation is an actual change in the DNA sequence. Translocations do not change the DNA; they just copy it to a different location, and in the case of fusion, the fused chromosome has the same DNA as the two original chromosomes.]

You are also correct that a change in the number of chromosomes is often detrimental, or even lethal. But doesn't *have* to be. If the translocated DNA is sufficiently intact, only copied to a different location, then the organism may have all the genes it needs to survive. And in rare cases, this may actually be *advantageous*, if it loses some superfluous or detrimental genes in the process.

A perhaps bigger question is how this organism with the fused chromosomes can mate with other members of its own species to pass this characteristic (the reduced number of chromosomes) onto offspring and spread it into the population.

The answer is that a difference in chromosome number (change in ploidy), by itself, is not a barrier to interbreeding. If the sequence of DNA is compatible (which would be the case of a member of the same species with a recently fused chromosome), then it apparently doesn't necessarily matter that this DNA is on a single chromosome on one parent, and two chromosomes on the other parent.

For example, wild horses have 33 pairs of chromosomes, and domestic horses have 32 pairs. Not only can they interbreed, but the resulting offspring are fertile.

So the answer is that, as long as the fused chromosome has all the important DNA, there doesn't *have* to be any detrimental effect, or any barrier to interbreeding.

Answer 2

You have to have most of the same number of chromosomes as the animal you are ,well, breeding with. Like tigers and lions breed and make ligers, but they have the same number of chromosomes. I dont think you can have two in difference, so yes, i think it would cause a mental illness or deformity.